Plasma surface modification of single-atom Fe active sites on nano-sized graphene platelets for advanced oxygen reduction reaction

Single-atom Fe-doped carbon nanographene demonstrates promising results as an oxygen reduction reaction (ORR) electrocatalyst in fuel cells and rechargeable metal-air batteries. In this study, plasma-in-liquid process was applied to modify the surface of graphene nanoplatelet (GNP) with iron (II) Phthalocyanine as the Fe precursor and mixed into 1-Methyl-2-pyrrolidone (solvent) for 20 minutes. During the plasma modification, tiny discharge bubbles were observed, where the emission of C₂, CH, CN, and H radicals were identified by the optical emission spectrum. From the high-resolution TEM and the corresponding EDS elemental mapping images, the isolated Fe were successfully anchored as single-atom on GNP between 0.50 ~ 0.82 at.%. The ORR activity greatly enhanced after plasma modification, where the single-atom Fe-doped GNP demonstrated extremely high onset potential (E_onset) and half-wave potential (E_1/2) of 0.95− 0.98 V and 0.87−0.90 V vs RHE, respectively. Most importantly, plasma-modified Fe-GNP catalysts showed superior ORR catalytic activity compared to other single-atom Fe with similar metal loading catalysts reported in recent studies. The unique plasma-induced chemical reaction via active radicals not only successfully anchored atomic Fe onto the surface but also created certain defects of GNP, which created a synergic effect on the ORR catalytic activities.